Delivery of a shock pulse during the vulnerable period of the cardiac cycle can induce fibrillation, providing the shock energy is greater than a patient-specific minimum value and less than a patient-specific maximum value. Such a shock pulse is generally referred to as a “T-shock” or “T-wave shock” because the time of the vulnerable period during the cardiac cycle generally corresponds to the T-wave of the ECG signal.
Patients receiving an implantable cardioverter defibrillator (ICD) typically undergo defibrillation threshold testing in order to ensure a reasonable certainty of successful defibrillation using shock pulse energies corresponding to the output capacity of the ICD. The DFT has been determined by inducing fibrillation through delivery of a shock during the T-wave, then delivering defibrillation shocks to verify successful defibrillation at shock energies at least a safety margin below the maximum ICD output. ICDs are typically connected to intracardiac leads which are used to deliver a sequence of pacing pulses and deliver a T-shock. A sequence of pacing pulses, for example, six to eight pacing pulses, is delivered in order to facilitate delivery of a shock pulse synchronized with the T-wave during a stable cardiac rhythm. The T-wave shock is synchronized to the T-wave for inducing fibrillation by delivering the shock at a predetermined shock interval following the last pacing pulse. If fibrillation is induced, the ICD will detect the fibrillation and deliver a test defibrillation pulse to terminate the induced fibrillation.
Subcutaneous ICD systems are emerging which do not utilize intracardiac leads for sensing cardiac signals and delivering defibrillation shock pulses. Such systems, however, may not include fibrillation induction capabilities. As such, a need exists to induce fibrillation without the use of intra-cardiac leads to allow defibrillation threshold testing, for example, during subcutaneous ICD system implantation.